Second that emotion

As he was approaching the end of an orotund sentence, Denis Healey was ferociously attacked. Michael Berkeley's cat had slunk upstairs to the recording studio and landed in the politician's ample lap: it began to crawl all over his body before finally, contentedly, sinking its claws into his genitals. "But Healey's such a pro," says Berkeley, "that his voice never faltered, in spite of the pain."

They were in the middle of recording an edition of Private Passions, Berkeley's musical-request programme, which starts a new season on Radio 3 on Saturday. The first series was broadcast a couple of years ago and the programme has steadily grown in confidence and status ever since, acquiring prestige and popularity as a buddleia attracts butterflies. This year it won the Broadcasting Press Guild's coveted prize, Radio Programme of the Year: fluttering around it now are hundreds of potential guests, eager for invitations.

It is not like Classic FM's weekend request slots, which are hosted by Susannah Simons, who is primarily an excellent news journalist. Nor is it like Desert Island Discs, though it belongs to the same family, and its appeal - the possibility of learning more about an individual by means of the sort of music he or she likes - is very similar. Sue Lawley and Michael Berkeley listen to and admire each other's programmes, and there are plans to add to each further elements of the other - immediately, by giving Private Passions a Sunday repeat, and, ultimately, by extending Desert Island Discs to an hour.

But they will never be the same. While Sue Lawley's conversations are biographical and probing, the focus of Private Passions is primarily - almost exclusively - musical. Those who are invited to appear need to have strong and informed musical tastes, and after that it doesn't much matter who they are. "I'll never have anyone who is famous just for being famous," says Berkeley firmly. "I'd infinitely prefer to have a taxi-driver, if he had something important to say." Then he adds, with typical generosity, that perhaps that's unfair; that one mustn't prejudge people; that even the most obnoxious can sometimes display a frailer side when discussing music.

In practice, of course, the guests are better known than most taxi-drivers. They are often actors, doctors, psychologists, artists and historians. Only a few musicians are chosen, as Radio 3 is very well-equipped with such people - they invite just enough to maintain a balance; but writers are particularly welcome. "It could be," says Berkeley, "that because writers are used to the problem of trying to express the inexpressible in words, they are better than most of us at talking about music."

Two of his most successful guests were Barry Humphries, who, as himself, displayed a deeply thoughtful, arcane and knowledgeable love of music, and, as Dame Edna Everage, provided the funniest programme broadcast anywhere last Christmas, her bizarre and recondite choices culminating in a stirring recording of herself singing "Why Do We Love Australia?" at the Albert Hall.

Barry Humphries didn't put on a glam frock when he arrived for the second time at the house in west London where the programmes are recorded. Instead, he told Berkeley not to look at him too often, then they put on the headphones and began. The presenter kicked off in the right direction by complimenting the Dame on her dress: "Thank you, Michael," she simpered graciously, "I could tell you liked it by the way you're fingering the hem." "We had to cut out quite a lot of that one in the end," he remembers, smiling: too saucy for Radio 3, we assume.

Yet one of his instructions to his guests is that they should forget all about the august demands of Radio 3 and include anything that is important to them. So Ian Hislop chose recordings from the Renaissance and a blast of Elvis, John Peel had Conlan Nancarrow alongside Vivaldi and the writer Adam Mars-Jones selected Captain Beefheart: "It sat in the programme very well," says Berkeley. The general bias is, however, towards the classical. Some things come up frequently, like operatic moments from Mozart and, more surprisingly, Strauss's Four Last Songs, though he and his producers do try to steer people away from such marker buoys.

An advantage to having the studio in his own house (where he lives with his wife, the literary agent Deborah Rogers, and their 11-year-old daughter Jessica) is that guests can feel completely relaxed. They play the music, in its entirety as often as possible, into the conversation, which can have interesting effects. Miller remarked "I like that bit" during a record and Berkeley was able to explain to him that, musicologically speaking, it was a similar harmonic sequence to another passage he had chosen, which delighted the academic in Miller.

Sometimes, just to hear a record is emotionally overpowering. Oliver Sacks had enjoyed watching Berkeley's face as it was gripped by terror during a passage from Don Giovanni but was then, himself, overcome by tears at the sheer consummate skill of the piece of Bach he had chosen next. What did Berkeley do when confronted by this weeping? "Oh, we just sat quietly for a minute or two," he says.

Besides being a sensitive interviewer, Berkeley is a distinguished composer. He confesses himself disappointed by the reluctance of people - otherwise intelligent, curious people - to take on the challenge of contemporary music: "They are ready to try to understand, say, a painter like Jackson Pollock or a "difficult" novelist, but fall at the first fence when it comes to music."

He thinks it might be because we've been brought up with a very simple tonal language: "We learn to recognise a sequence that satisfies us, and if we don't get it we feel uncomfortable - yet the magic is that if you do allow it space, it's like unlocking the door of a secret garden." And he points out that, though we shy away from it in concert halls, we are completely used to wonderfully strange music in films. "Just think of the shower sequence in Psycho - all those strings banging on the back of the bow near the bridge - wah! wah! wah!"

When attempting to explain these things himself, he often uses visual or narrative images. Other composers have different methods: "I remember once asking Boulez - a name to strike terror into many a heart - where his music came from, and instead of some complex mathematical equation, he gave me some records of Burundi drumming, and gamelan music, where people use phasing without musical training, completely naturally." He is most delighted when a guest can find a way of introducing contemporary music so as to make it easier for the listener. The satirist John Bird and the psychotherapist Adam Phillips were two of the stars in this little firmament.

Once or twice people have tried to choose some Michael Berkeley music for the programme, but he doesn't really like that. Modestly, he finds the thought "incestuous". Yet for those of us curious to learn more about his own style, he has agreed to allow the next series to be introduced by a fanfare he wrote some years ago for a series Piers Plowright, the great visionary producer, made about Chaucer. Listen out for it, on Saturday at noon: it could well become part of your life. Oh, and if he invites you to appear, beware of the cat.

`Private Passions' (R3) begins on Sat at 12noon (repeated Sun 14 Sept, 6.30pm), with Douglas Adams.